In external radiotherapy, the patient is treated with high-energy radiation e.g. generated by a linear accelerator (‘linac’). The linac could for example be mounted on a rotating gantry, but it could also be mounted to a robotic radiosurgery system. External radiotherapy is generally delivered by means of a plurality of daily treatment fractions. Radiation can be continuously delivered during a single fraction while rotating the gantry, which is called “volumetric modulated arc therapy” (VMAT) or only be delivered while the gantry is not moving and shut off while rotating the gantry (step-and-shoot treatment). For VMAT, the treatment time is typically in the range of 2-4 minutes per fraction while for step-and-shoot it is typically in the range of 5-15 minutes per fraction. For a good treatment outcome a balance should be found between giving enough radiation dose to the treatment target, while sufficiently sparing neighbouring organs at risk. Providing a too high radiation dose to an organ at risk is likely to result in unwanted side-effects.
A treatment plan, describing beam directions, shapes and intensities, depends on a patient's anatomy and on a treatment target (tumor type, tumor location, size, infiltration, etc.). For this purpose a volumetric image of the patient is acquired before treatment. This image is called a treatment planning image. For acquisition of the treatment planning image, the patient is positioned in the same way as on the treatment table. The planning image, generally a CT image, represents a ‘snapshot’ of the patient anatomy at a time point prior to treatment. In some cases, further images are acquired in order to improve the target delineation (MR or PET).
The treatment plan which is based on the snapshot of the patient anatomy cannot account for anatomical changes within the patient, e.g. due to internal organ motion. Such changes are generally grouped into inter-fraction and intra-fraction changes. Examples of inter-fraction changes are weight-loss and tumour shrinkage/growth. Examples of intra-fraction changes are breathing, bladder filling, muscular relaxation, couching, and others. To monitor intra-fraction motion, online images could be used, wherein “online” refers to imaging during the actual radiotherapy fraction. To deal with anatomical changes occurring after the planning image, the target definition is generally extended by a treatment margin. The resulting volume of the treatment target and this treatment margin is called planning target volume (PTV).
US 2011/0075807 A1 describes an adaptive imaging method of monitoring intrafraction target motion during radiotherapy. This method includes using simultaneous Mega-Voltage (MV) and Kilo-Voltage (KV) imaging to determine an initial 3D target position. 2D target position is monitored using MV imaging during radiotherapy delivery and is in combination with an online-updated characterization of target motion that are disposed to estimate if the target has moved beyond a 3D threshold distance.